Proximity detectors have been used in a variety of telephony applications. In one application, proximity detectors have been employed in telephones to determine the distance between the telephone handset and a user's ear. Determining whether the user's ear is close to the telephone handset allows set transitions to be made from a handset mode to a hands-free mode and vice versa. Also, this allows the acoustic output from the telephone handset loudspeaker in dual- mode mobile telephones to be limited. For example, U.S. Pat. No. 5,729,604 to Van Schyndel discloses an infrared proximity switch for use in a telephone to detect the proximity of a user's ear to the telephone handset. The proximity switch is used to switch the telephone from a hands-free mode to a handset mode when the telephone handset is brought near the user's ear.
In another application, proximity detectors have been employed in telephones to determine the proximity of the telephone handset to the telephone set. Determining whether the telephone handset is close to the telephone set also allows set transitions to be made from a handset mode to a hands-free mode and vice versa. Also, the proximity detector can be used as a hookswitch generator. For example, U.S. Pat. No. 3,109,893 to Burns discloses a proximity switch that permits a user to control a telephone without coming into physical contact with the telephone. The proximity switch is of the capacitive type and changes capacitance in response to the proximity of an object such as a user's hand.
In yet another application, proximity detectors have been employed in telephones to determine the proximity of a user's hand to the telephone. Determining whether a user's hand is close to the telephone allows the rate of adaptation or mode of operation of adaptive speaker telephones to be varied or altered thereby to alter acoustic coupling between the loudspeaker and the microphone. For example, U.S. Pat. No. 4,330,690 to Botros discloses a proximity switch for use in a telephone handset to detect the proximity of the telephone handset and the telephone set. The proximity switch includes an emitter coil in the telephone set and a pickup coil in the telephone handset. The proximity switch is operated by the inductive coupling of the emitter and pickup coils.
Although proximity switches have been used in telephones, there is a need to improve proximity switches to make them less obtrusive, more reliable, less costly and smaller. This latter requirement is especially important when proximity switches are to be used in mobile telephones.
It is therefore an object of the present invention to provide a novel ultrasonic proximity detector for use in a telephone device and a telephone device incorporating the same.